The simplest solution is to just remove the quantifier, and add a negative lookahead. Then the regex becomes:

    m[
      <%           # Opening delimiter.
      (?:  [^%]+   # Things that can't start one.
         | % (?!>) # % that aren't part of a closer
      )*           # As many non-closing-delims as you like.
      %>           # Closing delimiter.
    ]x
[download]

Backtracking no longer needed. (-: